Start-Up Sequences of an Offshore Integrated MMC MTDC System

With the technological development on wind power and voltage source converter (VSC) based modular multilevel converter (MMC), the integration of offshore wind energy into power grids using MMC HVDC, especially the multiterminal HVDC (MTDC) technique is considered to be a promising approach. This paper investigates the start-up of a doubly-fed induction generator (DFIG)-based offshore wind farm (OWF) integrated MMC MTDC system. Due to the fact that the AC network of an OWF is relatively weak, it is important to coordinate the start-up of the MTDC system and for the wind farm integration to achieve the system start-up in a stable manner. Based on the analysis of the control mode of the DFIG, the control modes of the MMCs are discussed and different start-up sequences are proposed. A 4-terminal MMC HVDC system with 1 terminal connected to an OWF is established on the real-time digital simulator (RTDS). The dynamic characteristics of the proposed start-up sequences are compared through real-time simulations. The characteristics and validity of the sequences are compared with a preferred start-up sequence proposed.